This invention relates to the art of producing thin-walled honeycomb structures from extrudable material such as particulate ceramic and/or metal batches and similar materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity. More particularly, the present invention relates to an improved extrusion die assembly for forming a honeycomb structure with an integral outer peripheral skin by modifying the outer peripheral face of a conventional extrusion die structure and providing flow control and thickness control means for producing the desired skin.
The prior art is replete with extrusion die and mask assemblies of different configurations for providing cellular structures with an outer skin; however, virtually all such assemblies require the use of specific dies and incorporate rather complex assemblies, and do not contemplate the unique concerns of the extrusion die apparatus of the present invention.
U.S. Pat. No. 4,349,329 relates to an extrusion device for producing honeycomb structures wherein a pooling zone is provided with batch material solely from the discharge slots, and such zone then supplies material to form an outer wall and thickened web portions between outer peripheral cells.
U.S. Pat. No. 4,381,912 discloses an extrusion die for forming a honeycomb structure wherein the skin material flows laterally through peripheral discharge slots alone, or through such slots and a gap, to knit with center webs and form a peripheral skin thereon.
U.S. Pat. No. 4,668,176 relates to an extrusion device for the production of honeycomb structures wherein a spring element controls the thickness of a passageway for the flow of skin material, and accordingly the thickness of the resulting skin, and a pivotal plate controls the flow of such skin material to such passageway.
Japanese Unexamined Patent Application, Publication 61-5915 relates to an extrusion die for forming honeycomb shaped material, wherein the rate of extrusion of the material adjacent the skin is controlled by the use of shorter rate-adjustment pins so as to match the rate of extrusion at the center of the die.
Conventional skin-forming processes of the prior art generally depended upon the wet, newly-formed substrate just emerging from the die to act as one of the forming surfaces to assist in creating an exterior skin. The second forming surface was usually a mask, contoured to the ultimate desired shape of the substrate. However, the skin extruded with such a process optimally formed at a thickness near or equal to the thickness of the internal webs of the substrate, and most attempts aimed at increasing the skin thickness resulted in the collapse of one or more rows of cells in the peripheral area of the webs.
In order to overcome the problems and complexities with the above-noted die assemblies of the prior art, it is an object of the present invention to provide an extrusion die assembly for forming thin-walled cellular substrates with an integral outer skin of greater thickness than the webs of the substrate so as to improve both wet strength and the structural integrity of the fired structure.